-----Original Message-----
From: Scott Kauling [mailto:[log in to unmask]]
Sent: Saturday, 8 December 2001 06:31
To: [log in to unmask]
Subject: [TN] PWB Fab Note Question - Diameter True Position


In IPC 2221 section 9.2.2 describes hole location tolerance, it is expressed
as "diameter about true position".  IPC T-50 defines "true position", not
diameter about true position.

My questions are:

        1. How does the diameter about come into play with the true
position?

  Think of the "True Position" as an ideal or perfect location with respect
to some datum (starting point). The tolerance on the location of that point
is a CIRCLE around that point (a diameter about the true position). Here's
where it gets a little complicated: that tolerance is usually expressed with
regard to "maximum material condition" (MMC). In the case of a hole being
drilled at that location, that means that tolerance will apply when the hole
is drilled at the smallest diameter (hence, the condition of having the
maximum material still left in place). As the hole is drilled to larger and
larger sizes, the difference between the MMC and the actual hole size may be
added to the positional tolerance.
  Here's an example:
* A hole is defined as located 1.000" in the "B" and 1.000" in the "C" from
some datum, with the bottom surface of the PCB defined as datum "A". The
positional tolerance on the hole is 0.006" MMC, ABC.
* The hole is defined as having a diameter of 0.125" +/- 0.004", and is
unplated (unsupported, in IPC jargon).
* The MMC of the hole would be 0.121" (the condition when the maximum
material is left in place). Applying the positional tolerance, if the hole
is drilled at 0.121" diameter, then it's center (it's location) must fall
within a circle of 0.006" around the ideal location.
* If the hole is drilled at 0.125", the difference between MMC and the hole
diameter can be applied to the locational tolerance. Adding 0.004" to 0.006"
gives 0.010". Therefore, that 0.125" hole would only need to fall within
0.010" of it's true position. The reason? Because the hole now has more
clearance area, and so can be located farther from it's ideal location  - by
exactly the amount that the clearance has been increased. The near edges of
both holes lie at the same point, no? The exception to this would be if
"regardless of feature size" (RFS) has been added to tolerance as a modifier
instead of MMC.

 Here's something else to think about:
* Imagine defining the location of a hole with a bilateral tolerance of +/-
0.005", instead of a positional tolerance. What's been defined is square
zone that is 0.010" on each side. Now imagine drilling a hole at one corner
of that square; 0.005" up and 0.005" to the left. That hole is NOT 0.005"
from the center. It is 0.007" from the center. That's true for all four
corners. That means you need to allow for the hole to be up to 0.007"
radially from it's ideal location. Since you need to allow for it, why not
define it that way? That's the reason for a circular tolerance zone. That
circle provides the fabricator with 57% more area to place the hole.


2. What tolerance of diameter about true position do you specify on your fab
prints

3. What is the typical capability of diameter about true position that PWB
fabricators hold?

These are almost the same question. We call out 0.006" positional tolerance
at MMC. We also follow the guidelines in IPC-2222 for tolerances on hole
diameters (supported and unsupported). You'll find these in sections 9.2 &
9.3. Call me if you have more questions.

Good Luck!

Michael Hiteshew
Lockheed Martin NE&SS Marine Systems
410-682-1259

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